Transformers typically contain dielectric fluids which act as insulators and also serve as coolants as well as suppressing arcing and corona formation under operation of the transformer. Because transformers are typically sealed devices that operate under conditions of elevated temperatures, transformer oils must be stable for prolonged periods of time. Transformers range from small devices such as capacitors to large devices in power generating facilities.
Transformer oils are formulated so that they meet or exceed certain specific, performance conditions. These conditions include a minimum pour point, a maximum kinematic viscosity and enumerated limits on interfacial tension, impulse breakdown strength, gassing tendency and levels of acid number and sludge produced in oxidation tests.
Currently, many transformers in service use naphthenic distillates as the basestock for formulating transformer oils. Typically the basestock is combined with an effective amount of an antioxidant additive, commonly a hindered phenol. As electrical equipment manufacturers develop more efficient electrical devices there will be a need for electrical oils that are more stable than oils based on naphthenic basestocks.
One approach has been to modify the basestock used in transformer oils. U.S. Pat. No. 6,790,386 describes the use of a dielectric oil containing a hydroisomerized isoparaffinic oil and a hydrogen donor compound. Such oils are stated to have negative hydrogen gassing properties, good oxidative stability and good low temperature performance. U.S. Pat. No. 5,167,847 describes a transformer oil prepared by solvent dewaxing a hydrocracked basestock.
Natural and synthetic esters have been used in certain transformer applications. Natural esters may be produced from natural products such as seeds. Synthetic esters are formed by esterifying fatty acids with alcohols. Such esters are environmentally friendlier and offer performance improvements such as higher flash points. They are limited in having inferior oxidative stability and poorer low temperature properties.
Synthetic oils have also been used for transformer oils. A common synthetic oil in transformer service is poly-alpha olefin (PAO). PAO's usually contain additives to yield products that have acceptable gassing properties.
Higher oxidation resistance can be achieved by use of paraffinic basestocks; however, paraffinic basestocks exhibit what is referred to as a positive gassing tendency. The gassing tendency of an oil is a measure of the rate at which hydrogen gas is either evolved or absorbed in an insulating medium when that medium is subjected to electrical stress sufficient to cause ionization. A positive gassing tendency indicates that hydrogen gas is given off, while a negative gassing tendency indicates that hydrogen gas is absorbed. A negative gassing tendency, or very low positive tendency, is desirable since it will minimize the build-up of hydrogen gas which could react with oxygen in the presence of a discharge spark to cause an explosion in the electrical device. Insulating oils shown to have gas absorbing characteristics have been used to advantage in reducing equipment failure, particularly in cables and capacitors. The gassing tendency of electrical oils is measured by test method ASTM D 2300. Oils that evolve hydrogen gas have a positive test value and those that absorb hydrogen gas have a negative test value.
It would be desirable to develop additives for transformer oils that would impart good gassing tendency and oxidative stability to transformer oils while having only minimal impact on viscosity and volatility.